1 files changed, 77 insertions, 0 deletions
diff --git a/packages/frontend/src/composables/use-scroll-position-keeper.ts b/packages/frontend/src/composables/use-scroll-position-keeper.ts
new file mode 100644
index 0000000000..b584171cbe
--- /dev/null
+++ b/packages/frontend/src/composables/use-scroll-position-keeper.ts
@@ -0,0 +1,77 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { throttle } from 'throttle-debounce';
+import { nextTick, onActivated, onDeactivated, onUnmounted, watch } from 'vue';
+import type { Ref } from 'vue';
+
+// note render skippingがオンだとズレるため、遷移直前にスクロール範囲に表示されているdata-scroll-anchor要素を特定して、復元時に当該要素までスクロールするようにする
+
+// TODO: data-scroll-anchor がひとつも存在しない場合、または手動で useAnchor みたいなフラグをfalseで呼ばれた場合、単純にスクロール位置を使用する処理にフォールバックするようにする
+
+export function useScrollPositionKeeper(scrollContainerRef: Ref<HTMLElement | null | undefined>): void {
+	let anchorId: string | null = null;
+	let ready = true;
+
+	watch(scrollContainerRef, (el) => {
+		if (!el) return;
+
+		const onScroll = () => {
+			if (!el) return;
+			if (!ready) return;
+
+			const scrollContainerRect = el.getBoundingClientRect();
+			const viewPosition = scrollContainerRect.height / 2;
+
+			const anchorEls = el.querySelectorAll('[data-scroll-anchor]');
+			for (let i = anchorEls.length - 1; i > -1; i--) { // 下から見た方が速い
+				const anchorEl = anchorEls[i] as HTMLElement;
+				const anchorRect = anchorEl.getBoundingClientRect();
+				const anchorTop = anchorRect.top;
+				const anchorBottom = anchorRect.bottom;
+				if (anchorTop <= viewPosition && anchorBottom >= viewPosition) {
+					anchorId = anchorEl.getAttribute('data-scroll-anchor');
+					break;
+				}
+			}
+		};
+
+		// ほんとはscrollイベントじゃなくてonBeforeDeactivatedでやりたい
+		// https://github.com/vuejs/vue/issues/9454
+		// https://github.com/vuejs/rfcs/pull/284
+		el.addEventListener('scroll', throttle(1000, onScroll), { passive: true });
+	}, {
+		immediate: true,
+	});
+
+	const restore = () => {
+		if (!anchorId) return;
+		const scrollContainer = scrollContainerRef.value;
+		if (!scrollContainer) return;
+		const scrollAnchorEl = scrollContainer.querySelector(`[data-scroll-anchor="${anchorId}"]`);
+		if (!scrollAnchorEl) return;
+		scrollAnchorEl.scrollIntoView({
+			behavior: 'instant',
+			block: 'center',
+			inline: 'center',
+		});
+	};
+
+	onDeactivated(() => {
+		ready = false;
+	});
+
+	onActivated(() => {
+		restore();
+		nextTick(() => {
+			restore();
+			window.setTimeout(() => {
+				restore();
+
+				ready = true;
+			}, 100);
+		});
+	});
+}